This invention relates to multi optical-fiber cables, and more particularly to a means and a method for optically coupling such cables to improve the efficiency of light propagation across said coupling.
The transmission of radiant energy along a plurality of fine optic fibers in a composite cable, sometimes referred to as "lightpipes", is becoming increasingly important. Employment of such cables in communication systems and the like have had limited suitability because of problems in manufacture, fragility in handling and installation, and particularly the lack of efficiency in coupling due to high transmission losses across junctions.
There has been a tendency in the art to connect the ends of optical cables in a similar manner as electrical cables. For example, in a splice connection, the bared ends of the light-conducting fibers of each cable are inserted and secured in a connector body half and the two halves secured together by a threaded sleeve or the like, such as shown in U.S. Pat. 3,455,625. This patent also teaches the use of an index matching material positioned between the bared ends of the fiber optic cable ends to provide a continuous optical transmission path between the cable ends. In other coupling devices the cable ends may be adhesively secured together with the connector half such as shown in U.S. Pat. 3,457,000.
The problem in using electrical-type connectors for coupling incoherent light-transmitting fibers is that in the former there was no need individually to align the axes of the ends of the conductors in one cable to the conductor ends in the other cable. However, in optic fiber cables, it has been found that very slight off-axis misalignments between potentially mating fibers being coupled will materially reduce the light propagated across the fiber-to-fiber junction. It can be shown from purely geometrical considerations that a 50% misalignment of the fiber ends will provide as little as 30% of the ideal transmission level. As with antenna propagation, angular deviations from the fiber axial direction will cause the received light to reduce as the square of its effective propagation gain level. A slight improvement in light transmission can sometimes be achieved by rotating one of the multi-fiber cables axially with respect to the other until the maximum transmission is achieved by observing measurements, but this effort is time consuming, and requires special tools and equipment difficult to implement in remote field installations. Furthermore, the improvement in light efficiency is not substantially significant.